Flexible displays made with a technology known as electronic ink, or E ink, are in the process of commercial development. While the early versions are expected to resemble simple displays that might sit by the side of a highway to warn of trouble ahead or might advertise specials at a convenience store, later versions are expected to lead to electronic books with paper-like pages and illustrations that move, newspapers that update themselves, reusable paper displays for cellular phones, disposable TV screens, and even electronic wallpaper.
There are two presently-known competing technologies: E Ink's electrophoretic displays and Xerox's gyricon spheres.
The electrophoretic displays are disclosed, for example, in U.S. Pat. No. 6,017,584, issued Jan. 25, 2000, and entitled “Multi-Color Electrophorectic Displays and Materials for Making the Same”, and in U.S. Pat. No. 6,067,185, issued May 23, 2000, and entitled “Process for Creating an Encapsulated Electrophoretic Display”.
Generally, an encapsulated electrophoretic display includes one or more species of particles that either absorb or scatter light. One example is a system in which the capsules contain one or more species of electrophoretically mobile particles dispersed in a dyed suspending medium. Another example is a system in which the capsules contain two separate species of particles suspended in a clear suspending fluid, in which one of the species of particles absorbs light (black), while the other species of particles scatters light (white). Other extensions are possible, including more than two species of particles, with or without a dye, etc. The particles are commonly solid pigments, dyed particles, or pigment/polymer composites.
The gyricon spheres are disclosed in a number of patents issued and assigned on their face to Xerox Corporation; an example of one such patent is U.S. Pat. No. 5,982,346, issued Nov. 9, 1999, and entitled “Fabrication of a Twisting Ball Display Having Two or More Different Kinds of Balls”.
The gyricon, also called the twisting-ball displayed, rotary ball display, particle display, dipolar particle light valve, ect., a technology for making a form of electric paper. Briefly, a gyricon is an addressable display made up of a multiplicity of optically anisotropic balls, each of which can be selectively rotated to present a desired face to an observer. Thus, in one version at least, the gyricon is a solid microsphere, hemispherically-colored black and white and having hemispherically-opposing zeta potentials. Each gyricon rotates within a dielectric oil-filled microcavity formed in the media upon exposure to an externally-applied electric field.
The primary disadvantage of both electrophoretic ink and the gyricon is poor contrast. Hemispherically-colored microspheres, or microcapsules, being fully three dimensional, have backside reflection and scattering that reduce the contrast of dark and white images reflected toward the observer. Additionally, both colorants are at least partially visible in microcapules containing both particulate and liquid colorant, independent of which colorant is electrophoretically moved toward the observer. This, too, negatively impacts image contrast.
The second disadvantage of both the electrophoretic ink and the gyricon solutions is limited image resolution. Both solutions are limited to practical microcapsule or microsphere diameters on the order of 20 to 40 micrometers. Electrophoretic ink microcapsules are limited by the need to microencapsulate sufficient pigmented colorant to provide reasonable color contrast and opacity within each microcapsule. Gyricon spheres are limited by thermal mass requirements to form microspheres from coalesced colored droplets in water. Microsphere diameters on the order of 5 to 10 micrometers are desired and common to toner colorant used in laser printers.
Each prior solution must use a low dielectric liquid (oil), rather than water. Water, being conductive, would collapse the electric field that otherwise allows electrophoretic movement of the colorants. The colorant switching time and voltage is dependent on oil viscosity, which is negatively impacted by lowered ambient temperature.
Finally, the prior art colorants have poor mechanical durability by virtue of their microcapsule composition. Microcapsule fabrication processes generally produce micron thin capsule walls, typically 10% of the capsule diameter, that are easily broken. This factor is why microcapsules are typically used to deliver encapulsated fluids upon application of external pressure or salvation (e.g., carbonless paper) The fragile nature of microcapsules makes them poorly suited for electronic paper applications where folding and surface contact is common.
Thus, what is needed is a molecular system that exhibits image contrast and mechanical durability commensurate with ink on paper, avoids chemical oxidation and/or reduction, permits reasonably rapid switching from a first state to a second, is reversible to permit real-time or video rate display applications, and can be used in a variety of optical display applications, such as e-ink.